1. Field of the Invention
The present invention relates generally to a peristaltic pump. More particularly, it relates to a blood pump including a rotor assembly having a single drive roller, a counter-balance support, and a preformed pump chamber or tube arranged in a generally cylindrical pump housing. The rotor assembly, the preformed tube the housing or a combination thereof, may be disposable,. The present invention finds particular utility in extracorporeal medical procedures, for example, where used in combination with a blood oxygenator as a heart/lung device for emergency procedures. Of course, the roller pump of the present invention may be found to have equal advantage in other procedures and applications.
2. Description of the Prior Art
Blood pumps for extracorporeal procedures are known. Generally, those procedures include withdrawing oxygen depleted blood from a patient, e.g., from a femoral vein of the patient, circulating the blood through a pump and oxygenator system, commonly called a heart/lung machine, and reintroducing the oxygenated blood into the patient, e.g., through the femoral artery of the patient. Known blood pumps generally include centrifugal pumps, screw pumps, turbine pumps, diaphragm pumps and roller pumps.
Of these, roller pumps provide particular utility in extracorporeal procedures. Generally, roller pumps are advantageous because the blood handling portion of the pump, which must be sterile, is simply a piece of tubing with a very low volume. The low volume tube is advantageous because the supply of blood available for surgical procedures may be limited, particularly for emergency procedures or pediatric procedures. Also, since a blood pump often is primed with a saline solution, the low volume tube minimizes any adverse consequences of hemodilution. Thus, the low volume tube facilitates quick, easy, sterile priming of the pump. In addition, the volume output of a roller pump per minute is proportional to the number of revolutions per minute of the roller. Therefore, the volume output of a roller pump is an easily controlled variable during extracorporeal circulation.
Roller pumps generally are described either as single roller, twin roller or multiple roller pumps. A single roller pump generally comprises a cylindrical housing in which a 360.degree. loop of tubing is inserted. Typically, a large single eccentric roller is disposed within the loop for rotation on a drive shaft. A cam structure typically is provided for radially adjusting the roller relative to the loop so as to close, or occlude, the lumen of the blood tube. Alternatively, the roller can be biased radially by a spring. In each case, rotation of the drive shaft and roller provides a steadily progressive compression of the loop that "milks" the fluid out of the tube, e.g., by peristaltic action.
A twin roller pump typically comprises a semicylindrical housing in which a tube is disposed to form a 180.degree. turn or U-shape. A pair of rollers are disposed at ,opposite ends of a rotor arm pivotally disposed on a drive shaft about a common axis with the 180.degree. U-turn of the tube. Each roller may be provided with a cam structure for adjusting the tube occlusivity setting, or a spring structure for biasing the roller against the tube. Rotation of the drive shaft rotates each roller about the common axis of the housing to provide a steadily progressive compression of the 180.degree. turn and to milk the fluid out of the tube, e.g., by peristaltic action. Thus, in a twin roller pump, each full rotation of the drive shaft provides two successive compression cycles, one for each of the twin rollers.
U.S. Pat. No. 4,179,249 (Guttman) describes a multiple roller pump. More particularly, it describes a peristaltic pump including a rotor assembly having three drive rollers. In the Guttman patent pump, a pair of reaction members (18,20) pivotally are mounted on a base plate (12) for movement between an open position and a closed position relative to a rotor assembly (14). The rotor assembly (14) includes a pair of support disks (44,46) and three drive rollers (48a,b,c) disposed therebetween. The reaction members each have: a cam surface or channel formed therein, and are releasably retained in the closed position by a locking plate (26). In operation, a compressible tube is disposed around the rotor assembly in a U-shaped configuration when the reaction members are in the open position. The reaction members then are closed so that the compression tube is registered in the channel. The rollers are mounted between the support disks such that their outer cylindrical surfaces define compression surfaces for rolling engagement with the compressible tube during operation of the pump. Rotation of the rotor assembly causes the rollers to progressively compress the U-shaped segment of the compressible tube to pump fluid therethrough, e.g., by peristaltic action. Thus, each full rotation of the rotor assembly provides three successive compression cycles, one for each of the three drive rollers (48a,b,c).
A drawback of conventional single roller pumps is kinking. Conventional roller pumps generally are designed to accommodate a standard or straight flexible tube drawn around in a turn or loop. However, a straight tube having a diameter of about 1/4 inch or greater tends to kink when rolled in a spiral turn or loop of a size suitable for a blood pump. Moreover, this tendency to kink increases when a roller is applied to engage the tube and occlude it.
A drawback of conventional twin and multiple roller blood pumps is size. A flexible tube has an associated recovery time, i.e., the period of time required for the tube to reopen after being pinched or occluded. Of course, as the tube reopens, it is refilled with fluid to be pumped out. Therefore, failure of the tube to substantially reopen reduces the output efficiency of the pump. For any given tube, increasing the number of drive rollers symmetrically disposed about the rotor assembly lowers the time between strokes, i,e., the time permitted for recovery and, thus limits the maximum rpm of the motor assembly for a desired efficiency. Therefore, in order to increase the maximum rpm, and thus the flow capacity, known twin and multiple roller blood pumps often are large, and often require occlusivity adjustment before use. Moreover, these pumps typically require a relatively long set-up time, e.g., up to 40 minutes, to insure safe operation.
A drawback of all conventional roller pumps in extracorporeal applications is hydroshock. Hydroshock is caused by a sudden change in localized pressure of the blood. Recent research performed in connection with artificial hearts suggests that hydroshock is a major etiology for hemolysis, with subsequent thrombus formation (See Henker & Murdaugh, "Effects of Pneumatic Artificial Heart Driver on the Rate of Isovolumic Pressure Rise," Artificial Organs, Vol. 12, No. 6, p. 519 (1988). Thrombus can cause stroke or other serious complications.
Hydroshock occurs in conventional roller pumps at the end of each pump cycle. As described above, a roller pump drives fluid by progressively compressing a tubular pump chamber. During operation of the roller pump, the outlet of the pump chamber (tube) communicates with the relatively high arterial pressure, and the inlet of the pump chamber communicates with the relatively low venous pressure. Accordingly, during each cycle of the roller, blood in the tube preceding the roller, i.e.., on the downstream side, is exposed to the relatively high arterial pressure. Blood in the tube succeeding the roller, i.e., on the upstream side, is at the relatively low venous pressure. When the roller disengages the tube at the end of the cycle, the preceding and succeeding portions of the tube come into fluid communication. Thus, blood in the preceding portion of the tube experiences a pressure drop, and blood in the succeeding portion of the tube is exposed to a sudden localized increase in pressure, or hydroshock. A sudden decrease in blood pressure may be harmful to the patient. Moreover, the hydroshock may cause hemolysis, thrombus or .other complications.
Another drawback of conventional blood roller pumps is tube occlusion during storage. As described above, a roller pump acts by progressively occluding, or pinching, a flexible rubber tube. As the roller progressively occludes the tube, the flexible tube progressively reopens behind the roller. However, during storage the roller is static. Thus, if the pump is stored with the tube resident therein, then the drive roller typically occludes a portion of the tube, and that portion tends to retain an occlusion "memory," whereby the portion always remains somewhat occluded, even when the pump later is taken out of storage and driven so that the roller progressively occludes the tube. This occlusion memory reduces the total volume of blood flow through the pump. More importantly, it makes it difficult for a clinician to accurately control the blood flow volume and pressure. Accordingly, conventional pumps typically are designed for storage without the tube resident therein, and require additional set-up time.